Our objective is to obtain new information on the mechanisms involved in replication and recombination of bacteriophage DNA. This information will be obtained primarily by electron microscopic measurements made on intracellular replicative and recombinational intermediates of bacteriophages lambda, P2, 186 and P4. These intermediates will also be isolated with their associated enzymatic complexes intact and electron microscopic partial denaturation will be used, where necessary, as a mapping procedure. We propose to further investigate bacteriophage DNA packaging and injection by a technique developed during the last grant period. The problems associated with a completely automatic electron microscopic DNA length measurement system will be investigated by computer simulation.